284 BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



HEMIPHRAGMA IRRASUM (Ulrich). 

 Text figs. 172, 173. 



Batostoma irrasa Ulrich, Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minnesota, 

 1886, p. 94. 



nemiphragma irrasum Ulrich, Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 

 1893, p. 299, pi. 24, figs. 5-19; Zittel's Textbook of Paleontology (Eng. ed.), 

 1896, p. 275, fig. 460. — Simpson, Fourteenth Ann. Rep. State Geologist ol 

 New York for the Year 1894, 1897, p. 592, figs. 190-193. 



This very abundant American Black River species is represented 

 in the Russian collection by a large zoarium which, as a comparison 

 of figures 172 and 173 will show, has a structure practically identical 



Fig. 172. — HEMIPHRAGMA IRKASXTM. /, VTEWS OF THE MINNESOTA TYPES, AFTER ULRICH. a AND 6, TWO 

 FRAGMENTS, NATURAL SIZE, SHOWING DIFFERENCE IN SIZE AND BRANCHING; C, SURFACE OF AN AVER- 

 AGE EXAMPLE, X9; d AND 6, TANGENTIAL SECTIONS, X18, EXHIBITING RANGE OF STRUCTURE NOTED IN 

 VARIOUS PARTS OF THE ZOARIUM; /, VERTICAL SECTION OF BRANCH, X9. BLACK RiVEE (DECORAH) 



SHALES, Minneapolis, Minnesota. 



with that of Ulrich's types. In fact, this specimen differs only in the 

 greater size of its branches, a difference due entirely to age. Ulrich 

 has given the following description of HemipTiragma irrasum: 



Zoarium consisting of small, subcylindrical, frequently and rather irregularly divid- 

 ing branches, commonly 5 or 6 mm. in diameter, but varying from 4 to 8 mm., the 

 latter extreme probably only when an extra layer of tubes has grown over the original 

 branch. Monticules wanting, but under fully matured conditions the surface is abun- 

 dantly spinulose. Zooecia with subangular apertures, and thin walls when young, 



